Investigators at Baylor College of Medicine have extensive experience and interest in gene therapy research. Bone marrow transplantation using murine models are a key experimental approach in preclinical testing of vectors and gene transduction protocols. At the present time the school does not have an appropriate instrument for the safe administration of graded doses of irradiation to small animals. Lethal total body irradiation (TBI) of mice has been done using a Gammacell 1000 (137)Cs source located in the laboratory of Dr. Robert Rich. This unit has a minimum dose rate that is approximately 2.5-fold greater than optimal. In addition, the exposure field is narrow leading to inhomogeneous dosage. Baylor College of Medicine is building a new specific pathogen-free (SPF) mouse facility. To comply with the strict isolation procedures required by this facility it will be necessary to place the irradiator within the new mouse unit. The irradiator will be an essential element in most of the gene therapy-related research projects at Baylor. Other research projects will also require access to this instrument including studies of T cell development, the selection of the T cell receptor repertoire, and biology of osteoclasts in src-deficient mice. The provision of this instrument within the strict biological containment mouse facility will allow rigorous compliance with NIH operating procedures and more accurate interpretation of crucial preclinical experiments.